1. Field of the Invention
This invention relates to high-temperature combustion of dry, green or damp biomass fuels such as wood, brush and bark chips, sawdust, peat, nut hulls, straw or logs, and the efficient utilization of the evolved heat so that the moisture present in the fuel is condensed and 85 to 95% of the high heat content of the fuel is reclaimed.
2. Description of the Prior Art
Smoke pollution from wood stoves is reaching alarming proportions throughout the world, and conventional sawdust, chip, and hogged fuel-burners suffer from the same symptoms of inefficient combustion. Relatively dry fuels are generally required for cleaner burning, but biomass fuels with 40% or higher moisture content (% of damp weight) are much more available in the form of logging residue, brush and agricultural waste, at a lower cost than seasoned cordwood, and lend themselves better to automatic continuous fuel feed. Combustion problems with wood and other biomass fuels have been generally due to not enough heat for drying and ignition, uncontrolled cycles of drying and ignition, with either excess air or insufficient air, too much fuel burning at once or too little, and incomplete mixing of air and fuel. At the low burn rates generally required for domestic use exhaust emissions increase dramatically, because of incomplete combustion, which causes creosote and soot buildup on heat exchangers, and in turn fire hazards and even more inefficient heat transfer. Prior solutions have involved first making the fuel as dry as practical and then burning it as hot as possible, often with the consequence that more heat is generated than is needed and too much fuel is consumed. Another partial solution has been to use electric blowers to increase draft and the efficiency of the heat exchangers, with the consequence that the system is inoperative during power outages.
Numerous patents have been issued for improved wood stove designs and a few for sawdust and chip burning devices, but none is similar in design and operation to the biomass-fueled furnace described here. The closest prior art publications are two magazine articles about my earlier research efforts in developing this burner, namely "The Grendel Report" by Larry Dobson, Alternative Sources of Energy, June, 1980; and "An Amazingly Efficient Sawdust Stove", The Mother Earth News Guide to Home Energy, Autumn, 1980. However, these articles relate to a furnace quite different from the one described here, being a downdraft design with vertical baffles rather than a cross-draft-up-draft design with horizontal baffles as described here, combustion air being heated by the combustion chamber walls rather than through hollow grates in the ignition chamber or by the air heat exchanger as described here, insulation being with dirt, rather than high-temperature ceramic fiber insulation as described here, and air and water heat exchangers of conventional design, rather than the improved counterflow models described here.